1. Field of the Invention
This invention relates to an electromagnetic radiation measuring apparatus for measuring an electromagnetic radiation from an operated unit such as an operated circuit board.
2. Description of the Prior Art
An electromagnetic radiation measuring apparatus comprising two-dimensionally arranged probe elements for receiving electromagnetic radiation and a switching unit for successively supplying a detection signal from one of probe elements to provide two-dimensional electromagnetic detection result is known.
Such a prior art electromagnetic radiation measuring apparatus is disclosed in Japanese Patent Publication No. 5-67184. This prior art electromagnetic radiation measuring apparatus comprises a wire loop antenna matrix on a multi-layer print circuit board, each wire loop antenna detecting an electromagnetic radiation and converting it into an electronic signal, and a switching circuit, including diodes, for successively outputting the electronic signal from one of the loop antenna.
FIG. 5 is a circuit diagram of a second prior art electromagnetic radiation measuring apparatus. FIG. 6 is a partial circuit diagram of a second prior art electromagnetic radiation measuring apparatus. In FIGS. 5 and 6, electromagnetic radiation measuring arrays 104 arranged in X direction. Each of electromagnetic radiation measuring arrays includes a plurality (generally, 20 to 40) of wire loop antennas 101 and a switching diode 103 forming an electromagnetic detection unit on a print circuit board arranged in Y direction. A transmission line 102 in each electromagnetic radiation measuring array is connected cathodes of respective diodes. An electromagnetic radiation is detected by a wire loop antenna 101 is converted into an electric detection signal which is supplied to the transmission line 102 through the switching diode 103, wherein a selection in Y direction is effected by applying a Y direction selection signal to a terminal 105 to turn on the corresponding switching diode 103. Therefore, the detection signal from the selected wire loop antennas is supplied to the transmission line 102 through the switching diodes 103. One end of each of the transmission lines 102 is terminated with a terminating element having a characteristic impedance (generally 50 ohms). The other ends of the respective transmission lines 102 are supplied to a signal selection portion 119 having a transmission line 121 and switching diodes 120. That is, the respective transmission lines 102 are coupled to the transmission line 21 extending in the X direction through the switching diodes 120. The transmission line 121 is so arranged as to be close to respective switching diodes 120.
When one of the switching diodes 120 is turned on and other switching diodes 120 is turned off to provide the selection in X direction. Therefore, the detection signal from one of loop antenna 101 is outputted.
In the second prior art, assuming that a switching diode 120a is turned on, because all other switching diodes 120 in the signal selection portion 119 are turned off, a portion A of the transmission line from the junction point between the switching diode 120a and the transmission line 121 to one end of the transmission line 121 acts as an open end transmission line. Therefore, This portion A shows a low impedance at a frequency corresponding to a wavelength .lambda.g, wherein EQU .lambda.g=4.times.A
Therefore, a transmission frequency characteristic of the signal selection portion 119 for selectively outputting one of outputs of the electromagnetic arrays is deteriorated. That is, this frequency limits the measurable high frequency (maximum frequency).
When it is desired to increase the maximum frequency, a positional measurable range is limited. Contrary, if it is desired to expand the positional measurable range, the maximum frequency is limited. For example, it is assumed that a length of the transmission line is 200 mm. A maximum length of the open end line of the transmission line 21 is about 150 mm. Then, if the print circuit board has a general dielectric constant, the maximum measurable frequency is about 230 MHz. However, it is general to require the maximum measurable frequency of the electromagnetic radiation up to 30 MHz to 1 GHz. Therefore, if the maximum measurable frequency is required to be 1 GHz, the length of the transmission line 21 should be about 70 mm. Then, this provides an insufficient positional range for a general size of a print circuit board to be measured.